Bangkok, Thailand — A UN panel has found “credible allegations” of war crimes committed by both sides during the final months of Sri Lanka’s civil war, which ended nearly two years ago with the battlefield defeat of the Tamil Tigers.

The panel’s findings contradict many of Sri Lanka’s assertions about the conduct of its troops, who are accused of deliberately shelling civilians caught in the war zone. The Liberation Tigers of Tamil Eelam (LTTE), whose leadership was wiped out at the end of the war in May 2009, is also cited for violations such as child recruitment and using civilians as human shields. Tens of thousands of Tamil civilians trapped behind enemy lines may have died during the fighting.

Sri Lanka’s government, which refused to cooperate with the panel, has already criticized its findings as “fundamentally flawed” and based on unverified claims. An executive summary of the report was leaked last week to a Sri Lankan newspaper after the UN sent a copy to the government.

"If proven, those most responsible, including Sri Lanka Army commanders and senior government officials, as well as military and civilian LTTE leaders, would bear criminal liability for international crimes," it said.

The strongly worded report, and Sri Lanka’s preemptive attacks, will put pressure on UN Secretary-General Ban Ki-moon, who appointed the panel last year. The UN also comes in for sharp criticism in the report for failing to speak out forcefully enough on civilian casualties during the fighting. Mr. Ban’s spokesman said Monday that the full report would be made public this week.

One option for Ban would be to set up a commission of inquiry as a possible first step toward a war-crimes prosecution, either at the International Criminal Court or another judicial body. This may depend on support in the UN Security Council, though analysts say that other mechanisms are at Ban’s disposal.

“We think that the secretary-general has the authority under the UN charter to initiate a fact-finding body that could deepen the panel’s work,” says Alan Kennan, a senior analyst at the International Crisis Group based in London.

Domestic inquiry was inadequate, critics say

Sri Lanka has argued that its own domestic inquiry into the war is sufficient and rejects its critics as misled by the LTTE. Over the weekend, President Mahinda Rajapakse vowed to lead mass protests on May 1 to defend national sovereignty. Last year, a government minister led a three-day siege of a UN compound in Colombo after the panel began its work.

However, the UN panel, which was led by Indonesia’s former attorney general, Marzuki Darusman, concluded that Sri Lanka’s fact-finding commission didn’t meet international standards of accountability. Human rights groups have complained that the commission affords no protection to witnesses and lacks independence from the government.

Gordon Weiss, a former UN official in Sri Lanka who has written a book on the conflict, said the panel had highlighted a “frontal assault” on international law that demanded accountability. He drew parallels with Srebenica, the 1995 massacre in the Bosnian War, and said the UN should bear some of the blame.

“The UN didn’t do enough. It wasn’t cohesive enough. It’s singled out [in the report] for failing to use the available casualty figures,” he says.

An internal UN report in April 2009 put the civilian death toll at 7,000. However, this report was never made public, in part because Sri Lankan officials strongly contested the figures. Mr. Weiss says that the final death toll may be as high as 40,000, as the suppressed report preceded the final assault on the LTTE base.

The UN panel found that the government had shelled no-fire zones where it had told civilians to gather. Troops also shelled hospitals, food distribution centers, and a beach strip where medical teams were evacuating the wounded. In all cases, these facilities had been identified to the government, it said.

Human rights groups say that a full independent investigation would help set the record straight for those caught up in the 26-year conflict, which not only pitted minority Tamils against the dominant Sinhalese but also fomented intraethnic fighting. Some accounts have put the final death toll at 100,000.

“We still don’t know all the details. We don’t know how many people died. We don’t know all the horrendous things that the LTTE did. This is what the people of Sri Lanka need to know if they are to put all this behind them,” says Mr. Kennan.

However, some Sri Lankan peace activists have warned that international pressure could play into the hands of a nationalist government that continues to use wartime emergency powers to crack down on dissent. In recent months, this has included criminal probes into the funding of aid groups that work on human rights, peace, and reconciliation, many of whom are critical of some government policies.